The Cyclotichium genus belonging to the Lamiaceae family is represented in Turkish flora by five species two of which are endemic. These species are C. niveum, C. origanifolium, C. glabrescens, C. leucotrichum, C. stamineum of which C. niveum and C. origanifolium are endemic.[1]Cyclotrichium species have been used for flavoring agent in food as well as herbal tea. Cyclotichium has been traditionally used as sedative, relaxant, carminative, and treatment of respiratory disorders.[2] Phytochemical investigation on C. niveum resulted in the isolation of isosakuranetin 7-O-rhamnoside, eriodictyol 7-O-glucoside, apigenin, apigenin 7-methyl ester, acacetin 7-O-rutinoside and triterpenoids vergatic acid, oleanolic acid, β-amyrin, and sitosterol.[3] Phytochemical studies including essential oils on Cyclotichium species were reported.[4],[5],[6],[7]

Reactive oxygen species (ROS) including free radical such as superoxide anion (O2·-), hydroxyl (OH·), peroxyl (ROO·), and nitric oxide (NO·) radicals are produced through oxidative process in eukaryotic cells. The human body maintains many protection mechanisms against oxidative stress, embracing antioxidant enzymes, and secondary metabolites. Exposure of environmental pollutant (cigarette smoke, pesticides, smog, UV radiation), the secondary metabolites production of mammalian become insufficient and then the excess of free radicals can damage to the cell membrane in chain reaction leading to degenerative diseases like Alzheimer disease, cardiovascular disease, ageing, cataracts, arteriosclerosis, diabetes mellitus, and DNA damage leading to carcinogenesis.[8] Recently, researches have focused on finding natural antioxidants for use in foods, cosmetics, and medicine to replace synthetic antioxidants due to their carcinogenicity.[9] The antioxidant phytochemicals particularly phenolic compounds exist in fruits, vegetables, and medicinal plants have gained the significant attention for having the potency to prevent the human diseases.[10]

Flavonoids, revealing significant and beneficial medicinal effects on human health, are one of the most common polyphenols. The extensive biological and pharmaceutical properties of flavonoids, such as antiinflammatory, antibacterial, immune-stimulating, antiviral, antioxidant, and anticancer, have been reported.[10],[11],[12] They are mostly found in fruits, vegetables, teas, and cocoa; therefore, an important quantity is consumed in our daily diet. It is generally accepted that flavonoids, as well as other polyphenols, play a significant role in protecting plants from both insect and mammalian herbivory.[13]

Although essential oil researches have been conducted on C. origanifolium,[14],[15] no work has been reported on this species regarding the bioassay-guided isolation and identification of secondary metabolites and investigation of antioxidant activities. Herein, bioassay-guided isolation and identification of isosakuranetin (1), eriodictyol (2), luteolin (3), naringenin (4), and apigenin (5) were achieved and their antioxidant activities were presented.

The plant material was collected from Hatay, Samandag, Turkey. After identification of plant by Prof. Dr. Ahmet Ilcim, Department of Biology, Mustafa Kemal University, a voucher specimen was deposited in the Herbarium of Biology Department (MKUH 1208).

Extraction and isolation

Aerial part of the plant materials were powdered by liquid nitrogen and then were boiled in water, afterwards extracted with hexane, ethyl acetate, and n-butanol successively. A portion of water solution was lyophilized to yield the water extract, besides the hexane extract, ethyl acetate extract, and n-butanol extract. Antioxidant activity tests were performed for these extract of which the EtOAc extract exhibited the most antioxidant activities [Table 1]. Therefore, chromatographic techniques were applied for EtOAc extract to isolate the bioactive compounds. The HPLC chromatogram of ethyl acetate extract showed that it consisted of five major products [Figure 1]. After Sephadex LH-20 (200 g) was kept in MeOH for 24 h, it was filled into the glass column (50 cm × 100 cm). EtOAc extract (4 g) was dissolved in MeOH (10 mL), then it was subjected to column chromatography with sephadex LH-20 as an adsorbent, eluted by MeOH with the flow rate as 0.1 mL/min. One hundred fractions (10 mL each) was collected. The fractions including the same compounds were combined and named as 0-30 (fatty acids), S1 (frs 31-51), S2 (frs 52-55), S3 (frs 56-60), S4 (frs 61-65), and S5 (frs 66-100). Semipreparative HPLC analysis was applied for S2 (frs 52-55) including the most secondary metabolites to isolate the bioactive flavonoids. Twenty milligram sample (S2) was dissolved in DMSO (1.0 mL) to obtain stock solution to be analysis. Two hundred microliter of this solution was injected to HPLC column, repeated 20 times to attain enough compounds for spectroscopic analysis (NMR) and antioxidant assays. Isosakuranetin (1), eriodictyol (2), luteolin (3), naringenin (4), and apigenin (5) were isolated, respectively.

DPPH free radical scavenging effect was executed according to the reported method.[16] To a DPPH solution (0.135 mM) in ethanol (1.0 mL) was added the samples solution at different concentrations (40, 80, and 160 μg/mL). The ethanol was added for final solution to be 4.0 mL. After the strongly mixing, the reaction mixture was incubated at room temperature in dark for 30 min. The absorbance measurement was executed at 517 nm on a spectrophotometer. Lower absorbance of the mixture revealed the higher free radical scavenging activity. The following equation was used to evaluate the scavenging ability of DPPH·.

DPPH· scavenging effect (%) = [(Ac-As)/Ac] × 100

in which Ac is the control absorbance and As is the samples absorbance.

ABTS radical cation scavenging effect

The treatment of ABTS (2.0 mmol/L) in water with potassium persulfate (2.45 mmol/L) at room temperature in dark for 4 h yielded the ABTS cation radical. ABTS+· was diluted with sodium phosphate buffer (0.1 mol/L, pH 7.4) to measure the absorbance at 734 nm. The reactions of ABTS+· solution (1.0 mL) with samples solution in ethanol (3.0 mL) at different concentrations (5-40 µg/mL) were executed. The inhibition was calculated at 734 nm for each concentration. The scavenging activity of ABTS+· was calculated by the given equation:

ABTS+· scavenging effect (%) = [(Ac-As)/Ac] × 100

in which Ac is the initial concentration of ABTS+· and As is the remaining concentration in the sample.[17]

Ferric ions (Fe3+) reducing antioxidant power assay (FRAP)

The reactions of extracts and compounds at different concentrations (5–40 µg/mL) in distilled water (1.0 mL) with sodium phosphate buffer (2.5 mL, 0.2 M, pH 6.7) and potassium ferricyanide [K3Fe(CN)6] (2.5 mL, 1%) were performed. The reaction mixture was incubated at 50°C for 20 min. Trichoroacetic acid (2.5 mL, 10%) was added to the mixture. Then distillated water (2.5 mL) and FeCl3 (0.5 mL, 0.1%) were added to the 2.5 mL of this solution. Absorbance was measured at 700 nm in a spectrophotometer.[18] Absorbance increase of the mixture revealed an increase of reduction capability.

Determination of total phenolic compounds

Total soluble phenolic compounds in extracts were determined with Folin-Ciocalteu reagent using gallic acid as a standard.[19] Briefly, 1.0 mL sample solution in methanol (1.0 mg/mL) was mixed with distilled water (45 mL) in a volumetric flask and 1.0 mL of Folin-Ciocalteu reagent was added to the reaction mixture. Sodium carbonate (3 mL, 2%) was added after 5 min later, and then the mixture was kept for 2 h with shaking. The absorbance measurement was executed at 760 nm in a spectrophotometer. The concentrations of total phenolic compounds in the sample were determined as micrograms of gallic acid equivalent.

Ethyl acetate extract revealing the most antioxidant effect as well containing high phenolic contents was subjected to chromatographic techniques to isolate isosakuranetin (1), eriodictyol (2), luteolin (3), naringenin (4), and apigenin (5) [Figure 2]. 1H NMR and 13C NMR spectral data of isolated compounds were given in [Table 2] and [Table 3], respectively. Isosakuranetin (1) is the first compound isolated from ethyl acetate extract by sephadex LH-20 column chromatography as well as semipreperative HPLC. Its molecular formula was determined as C16H13O5 by LC-TOF/MS (m/z 285.0763 [M-H]-) (calcd. 285.0741). 13C spectrum revealing the presence of one methyl, one methylene, seven methines, six quaternary carbons, and one carbonyl carbon accorded with the structure. In 1H NMR spectrum, the signal observed at δ 5.51 as doublet of doublet with vicinal coupling (J = 2.8 Hz, 12.6 Hz, H2), at δ 3.28 as doublet of doublet with germinal and vicinal coupling (J = 12.6 Hz, 17.0 Hz, H3α) and at δ 2.71 as doublet of doublet with germinal and vicinal coupling (J = 2.9 Hz, 17.0 Hz, H3β), at δ 5.88 (brs, H6), δ 5.89 (brs, H8) revealed the proposed structure. The observation of the cross peaks in HMBC spectrum from H2 to C-1', C-2', and C-6' revealed the attachment of phenyl group at C-2 position.[20] Eriodictyol (2), second compound isolated by chromatographic techniques, is a well-known bioactive compound existing in many medicinal plants. Its molecular formula was established as C15H11O6 by LC-TOF/MS (m/z 287.0555 [M-H]-) (calcd. 287.0556). The 13C NMR spectrum along with DEPT experiments exhibited one methylene, six methines, seven quaternary carbons, and one carbonyl carbon. In the 1H NMR spectrum, the signals appeared at δ 3.07 (dd, 12.6 Hz, 17.4 Hz) and δ 2.71 (dd, 3.1 Hz, 17.4 Hz) belonged to the diastereotopic protons bonded at C-3. The spectral data in the literature were also supported the proposed structure.[21] Another compound isolated as white solid was determined to be liteolin (3), C15H9O6, representing 11 degrees of unsaturation. LC-TOF/MS analysis indicated the molecular ionic peak at m/z 285.0394 [M-H]- (calcd. 285.0399). The 13C NMR spectrum as well as DEPT experiments revealing six methines, eight quaternary carbons, and one carbonyl carbon accorded with the structure. The comparison of spectral data with the literature confirmed the structure.[21] Narigenin (4), a well-known flavonoid possessing a broad spectrum of biological effects, was isolated as a fourth compound. Its molecular formula was determined as C15H11O5 by LC-TOF/MS (m/z 271.0612 [M-H]-) (calcd. 271.0606). The observation of one methylene, seven methines, six quaternary carbons, and one carbonyl carbon in 13C NMR spectrum suited with the structure. The characteristic signals appeared at δ 5.30 (dd, 2.9 Hz, 13.0 Hz, H2), δ 3.08 (dd, 13.0 Hz, 17.1 Hz, H3α), δ 2.67 (dd, 2.9 Hz, 17.1 Hz, H3β), δ 5.90 d (d, 2.2 Hz, H6), and δ 5.89 d (d, 2.2 Hz, H8) fitted with thestructure.[22] The last compound isolated from C. origanifolium was apigenin (5). The comparison of spectral values with those reported in the literature confirmed the proposed structure.[23] Due to the bearing acidic protons, the isolated compounds revealed the excellent antioxidant activities in all assays [Table 4]. These flavonoids have capable of scavenging free radicals by donating of electrons or protons to the radicals and also have the ability of inhibition lipid peroxidation [Table 2]. The carbonyl group at C-4 and a double bond between C-2 and C-3 are also important features for high antioxidant activity in flavonoids.[24]